The present invention relates generally to x-ray imaging systems. More particularly, the present invention relates to systems and methods of adjusting focal spot positioning relative to a target within an imaging tube.
Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, pass through an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
In computed tomography (CT) imaging systems, the x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. The x-ray source typically comprises an x-ray tube that emits the x-ray beam at a focal point. In order to generate the x-rays, a large voltage potential of approximately 150 kV is created across a vacuum gap between a cathode and an anode allowing electrons, in the form of an electron beam, to be emitted from the cathode to a target portion of the anode. In the releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target at a focal spot, whereby they are abruptly slowed down, directed at an impingement angle, α, of approximately 90°, to emit x-rays through a CT tube window.
The cathode or electron source is typically a coiled tungsten wire that is heated to temperatures approaching 2600° Celsius. The electrons are accelerated by an electric field imposed between the cathode and the anode. The anode, in a high power x-ray tube designed for current CT devices, is a tungsten target having a target face, that rotates at angular velocities of approximately 120 Hz or greater.
The focal spot has an associated location on a surface of the anode, often referred to as the focal track. The focal spot location is controllably translated within the x-ray imaging tube in order to perform a double sampling technique, which is utilized to improve modulation transfer functions (MTF) in the CT system. Double sampling is accomplished in conventional imaging systems by adjusting focal spot positioning on the target or surface of the anode, electronically without mechanical motion, via use of deflection coils or plates within an x-ray tube. The deflection coils and plates deflect an electron beam by creating either a local magnetic or an electrostatic field.
To perform double sampling, the focal spots are generally wobbled between two positions on the target in the direction tangent to the focal track. While this two-point wobbling can greatly improve image quality and resolution in resulting CT images, it also generates tremendous heat along the focal track of the anode. The buildup of this heat on the focal track generated by the wobbling focal spot can result in temperatures of greater than 3000 degrees Celsius, which can lead to reduction of x-ray tube performance and peak power capability by, for example, focal track melting, high voltage instability in the x-ray tube, or early life radiation output drop-off.
The heat generated at the focal spot is dependent on a number of factors such as the size of the focal spot, the direction of the wobbling, and the transition time and/or deflection distance between the two points. As such, various methods have been employed in the prior art in an attempt to lower these very high focal spot temperatures created by two-point wobbling. In order to combat the negative effects resultant from the high focal spot temperatures, many current designs significantly lower power levels for generating the x-rays. Other designs have attempted to lower the focal spot temperatures at the focal track by increasing the target rotation speed, increasing the focal spot size, increasing the deflection transition time between the two points in the wobble, or reducing the power capability of the x-ray tube.
Therefore, a need exists for reducing focal spot temperatures along a focal track on a target anode, without compromising optimal performance criteria of the x-ray source. That is, it would be desirable to design an apparatus and method for reducing focal spot temperatures on a target anode without the current associated needs to lower power levels for generating the x-rays, to increase the target rotation speed, to increase the focal spot size/spot radius, or to increase the deflection transition time.